The National Institute of Child Health and Human Development (NICHD) serves as the lead agency for the National Children's Study (NCS), a longitudinal cohort study that will examine the effects of environmental influences on the health and development of approximately 100,000 children. Environment is broadly defined to include biological, physical, chemical, and social/cultural influences on children's health and development. The goal of the Study is to provide information that will ultimately lead to improvements in the health, development, and wellbeing of children. Enrolled children will be followed from before bi1th until the age of 21 years. Additional background about the Study is available on the Study's Website (www.nationalchildrensstudy.gov).